About baseball ticketing in Japan: Are my sentence natural English? Can you correct them please?
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You can make a reservation using Internet.
Unfortunately, most of the official websites of the baseball teams have their ticket purchasing services only in Japanese. The teams also outsource the ticket purchasing services to other websites of ticket agencies, but their websites are only in Japanese too.

If you have helpers who can read and write Japanese, please ask them and make a reservation with them using Internet. However, some of the websites require your registration as the website’s member.

Does this sound natural?

Your English is really strong, and if you hadn't asked for corrections, I probably would have left this alone. Your English is good.
I do, however, have a couple suggestions to make it sound more natural and flow better.
1) Change "unfortunately" to "however". You need to show contrast between two ideas in this case.
2) "The teams also outsource to other agencies, and their websites are only in Japanese too." The subject has already been established, and repeating topics in English gets tiresome to read after a while.
3) "If you have friends who can read and write in Japanese, please ask them to help you make a reservation online." Just a word choice preference here.
4) "However, keep in mind that some of the websites require you to register as a member." Again, just a word choice preference.

Hope this helps! Your English is really good ^-^